Semiconductor device having improved crystal orientation

ABSTRACT

Nickel is introduced to a peripheral circuit section and a picture element section on an amorphous silicon film to crystallize them. After forming gate electrodes and others, a source, drain and channel are formed by doping impurities, and laser is irradiated to improve the crystallization. After that, electrodes/wires are formed. Thereby an active matrix type liquid crystal display whose thin film transistors (TFT) in the peripheral circuit section are composed of the crystalline silicon film crystal-grown in the direction parallel to the flow of carriers and whose TFTs in the picture element section are composed of the crystalline silicon film crystal-grown in the direction vertical to the flow of carriers can be obtained.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device using TFTs (thinfilm transistor) mounted on an insulating substrate such as a glass andmore particularly to a semiconductor device utilizable for an activematrix type liquid crystal display.

2. Description of the Related Art

A semiconductor device having TFTs on an insulating substrate such as aglass is known to be utilized in an active matrix type liquid crystaldisplay, image sensor and the like using such TFTs for driving pictureelements.

Generally a thin film silicon semiconductor is used for the TFT used insuch devices. The thin film silicon semiconductor may be roughlyclassified into two semiconductors; those composed of amorphous silicon(a-Si) semiconductor and those composed of silicon semiconductor havinga crystallinity. The amorphous silicon semiconductor is most generallyused because its fabrication temperature is low, it can be fabricatedrelatively easily by a vapor phase method and it has amass-producibility. However, because it is inferior as compare to thesilicon semiconductor having a crystallinity in terms of physicalproperties such as an electrical conductivity, it has been stronglydemanded to establish a method for fabricating a TFT composed of thesilicon semiconductor having a crystallinity to obtain a fastercharacteristic. By the way, as the silicon semiconductor having acrystallinity, there are known to exist a polycrystal silicon,microcrystal silicon, amorphous silicon containing crystal components,semi-amorphous silicon having an intermediate state betweencrystallinity and amorphousness.

The following method is known to obtain those thin film siliconsemiconductors having a crystallinity: (1) directly form a film having acrystallinity, (2) form an amorphous semiconductor film and crystallizeit by energy of laser light, and (3) form an amorphous semiconductorfilm and crystallize it by applying thermal energy.

However, it is technically difficult to form a film having favorablephysical properties of semiconductor on the whole surface of a substrateby the method of (1). Further, it has a problem in terms of cost thatbecause its film forming temperature is so high as more than 600° C., alow cost glass substrate cannot be used. The method (2) has a problemthat its throughput is low because an irradiation area is small when aneximer laser which is presently most generally used is used. Further,the laser is not stable enough to homogeneously treat the whole surfaceof a large area substrate. Accordingly, it is thought to be a nextgeneration technology. Although the method (3) has a merit that itallows to accommodate with a large area as compare to the methods (1)and (2), it is also necessary to apply such a high temperature as morethan 600° C. as the heating temperature. Accordingly, the heatingtemperature needs to be reduced in a case of using a low cost glasssubstrate. In particular, because the screen of present liquid crystaldisplay is enlarged more and more, a large size glass substrate needs tobe used accordingly. When such a large size glass substrate is used, itscontraction and strain caused during the heating process indispensablein fabricating the semiconductor produce a large problem that theyreduce an accuracy of mask positioning and the like. In particular,because the strain point of the 7059 glass which is presently mostgenerally used is 593° C., it deforms largely by the conventionalheating crystallization method. Further, beside the problems concerningto the temperature, it takes more than tens of hours as the heating timerequired for the crystallization in the conventional process, so thatsuch time needs to be shortened.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to solve theaforementioned problems by providing a process which realizes both thereduction of the temperature necessary for the crystallization and theshortening of the heating time in a method for fabricating a thin filmcomposed of silicon semiconductor having a crystallinity using a methodfor crystallizing a thin film composed of amorphous silicon by heating.The silicon semiconductor having a crystallinity fabricated by using theprocess of the present invention has physical properties equal to orsuperior than those of the silicon semiconductor fabricated by the priorart and is usable for an active layer region of TFTs. By using thistechnique, TFTs having necessary characteristics can be formedselectively on a substrate.

The inventors of the present invention perform the following experimentsand study concerning to the method of forming a silicon semiconductorfilm by a CVD method or sputtering method and crystallizing the film byheat.

When, after an amorphous silicon film is formed at first on a glasssubstrate, a mechanism of crystallizing the film by heating is studiedthrough experiments, it is recognized that the crystal growth beginsfrom the interface between the glass substrate and the amorphous siliconand it proceeds in a columnar shape in vertical to the surface of thesubstrate if the thickness of the film is more than a certain value.

The above phenomenon is considered to have been caused because crystalnuclei (seeds) which would become bases of crystal growth existed at theinterface of the glass substrate and the amorphous silicon film and thecrystals grow from the nuclei. Such crystal nuclei are considered tohave been impurity metal elements and crystal components (crystalcomponents of silicon oxide is considered to be existing on the surfaceof glass substrate as it is called as a crystallized glass) which areexisted on the surface of the substrate in a very small amount.

Then the inventors consider that it is possible to lower thecrystallization temperature by positively introducing the crystalnuclei. In order to confirm that effect, the inventors try experimentsby forming a film of a very small amount of another metal on the glasssubstrate, forming a thin film composed of amorphous silicon thereon andthen heating and crystallizing it. As a result, it is confirmed that thecrystallization temperature reduces when several metals are formed onthe substrate and therefore it is presumed that crystal grows centeringon foreign materials as the crystal nuclei. Then the inventors study themechanism in more detail on the plurality of impurity metals forreducing the temperature. The plurality of impurity elements are Ni, Fe,Co, Pd and Pt.

It can be considered that the crystallization has two stages; an initialstage of producing a nucleus and a stage of crystal growth from thenucleus. While the speed of the initial nucleus production stage may beobserved by measuring a time until a spot microcrystal is produced in afixed temperature, this time is shortened any time when an amorphoussilicon thin film is formed using the impurity metals as the base andthe effect of the introduction of the crystal nucleus on the lowering ofthe crystallization temperature is verified. Further, unexpectedly, itis observed that the speed of the crystal growth after the production ofnucleus also remarkably increases in the crystallization of theamorphous silicon thin film formed on a certain metal when the growth ofthe crystal grain after the production of nucleus is studied by varyingheating time. Although this mechanism is not clarified yet, it ispresumed that some catalytic effect is acting.

In any case, it is found that when the thin film composed of amorphoussilicon is formed on the film of a very small amount of a certain metalformed on the glass substrate and is then heated and crystallized, asufficient crystallinity can be obtained due to the two effectsdescribed above at a temperature less than 580° C. and in about 4 hourswhich have been impossible in the past. Nickel has the most remarkableeffects among impurity metals having such effects and is an elementselected by the inventors.

How nickel is effective will now be exemplified. Although more than 10hours of heating time is necessary in crystallizing a thin film composedof amorphous silicon formed by a plasma CVD method on a substrate(Corning 7059 glass), on which a thin film containing a very smallamount of nickel is not formed, by heating in a nitrogen atmosphere at600° C., the same crystallization state can be obtained by heating at580° C. for about 4 hours when the thin film composed of amorphoussilicon formed on the substrate on which the thin film containing a verysmall amount of nickel is formed is used. By the way, Ramanspectroscopic spectrum is used in the judgment (determination) of thecrystallization at this time. It can be seen that the effect of nickelis very great even only from this fact.

As it is apparent from the above explanation, it is possible to lowerthe crystallization temperature and to shorten the time required for thecrystallization when the thin film composed of amorphous silicon isformed on the thin film of a very small amount of nickel. Now a moredetailed explanation will be made assuming that this process is used infabricating TFTs. By the way, the nickel thin film has the same effecteven if it is formed on the amorphous silicon film, not only on thesubstrate (lower side of amorphous silicon film), and in the case of ionimplantation or plasma treatment as described later in detail.Accordingly, such a series of process shall be called as "nickelmicro-adding". Technically it is also possible to perform the nickelmicro-adding during when the amorphous silicon film is formed.

At first, the method of nickel micro-adding will be explained. Themethod of forming the thin film of a small amount of nickel on thesubstrate and forming the film of amorphous silicon thereafter and themethod of forming the film of amorphous silicon at first and forming thethin film of a small amount of nickel thereon have the same effect oflowering the temperature by adding a small amount of nickel. Further, itis clarified that any of the methods of sputtering method, vapordeposition method, spin coating method and a method using plasma may beused in forming the film. However, when the thin film containing a smallamount of nickel is formed on the substrate, the effect is moreremarkable when a thin film (base film) of silicon oxide is formed onthe substrate and then the thin film of a small amount of nickel isformed on the base film rather than when the thin film of a small amountof nickel is formed directly on the 7059 glass substrate. It isimportant for the low temperature crystallization of the presentinvention that silicon and nickel directly contact and it is consideredthat components other than silicon may disturb the contact or reactionof the both in the case of the 7059 glass.

Further, as for the method of nickel micro-adding, it is confirmed thatalmost the same effect can be obtained by adding (introducing) nickel byion implantation, other than the methods of forming the thin filmcontacting above or under the amorphous silicon. For nickel, it isconfirmed that the temperature can be lowered when an amount of morethan 1×10¹⁵ atoms/cm³ is added. However, because a shape of peak ofRaman spectroscopic spectrum becomes apparently different from that ofsimple substance of silicon when the added amount is more than 5×10¹⁹atoms/cm³, an actual usable range is considered to be from 1×10¹⁵atoms/cm³ to 1×10¹⁹ atoms/cm³. When the nickel concentration is lessthan 1×10¹⁵ atoms/cm³, the action as a catalyst such as nickel for thecrystallization decreases. Further, when the concentration is more than5×10¹⁹ atoms/cm³, NiSi is locally produced, losing the characteristicsof semiconductor. In the crystallized state, the lower the nickelconcentration, the more favorably the semiconductor may be used.

Next, the configuration of the crystal when the nickel micro-adding isperformed will be explained. It is known that when no nickel is added,nuclei are produced at random from the crystal nuclei at the interfaceof the substrate and the like, that the crystals grow at random from thenuclei until a certain thickness and that columnar crystals in which(110) direction is arranged in a direction vertical to the substrategenerally grow in a thicker thin film as described above and an almostuniform crystal growth is observed across the whole thin film as amatter of course. Contrary to that, when a small amount of nickel isadded, the crystal growth is different at a region into which the nickelis added and at the surrounding section. That is, it is clarifiedthrough pictures of a transmission electron beam microscope that in theregion into which nickel is added, the added nickel or a compound ofnickel and silicon become the crystal nucleus and columnar crystal growsalmost vertical to the substrate similarly to one into which no nickelis added. It is also confirmed that the crystallization proceeds in alow temperature also in the surrounding region where no nickel is added.A peculiar crystal growth that the direction vertical to the substrateis arrayed in (111) in that portion and needle or columnar crystal growsin parallel to the substrate, is seen. It is observed that some largecrystals among the crystals grown in the lateral direction parallel tothe substrate grow as long as several hundreds micron from the regionwhere a small amount of nickel is added and it is found that the growthincreases in proportional to the increase of time and rise oftemperature. For example, a growth of about 40 micron is observed inheating at 550° C. for 4 hours. Further, it is clarified that the largecrystals in the lateral direction are all single-crystal like accordingto pictures taken by the transmission electron beam microscope. When thenickel concentration is examined at the portion where a small amount ofnickel is added, at the nearby lateral growth portion and at the furtherdistant amorphous portion (the low temperature crystallization does notoccur at the considerably distant portion and the amorphous portionremains) by SIMS (secondary ion mass spectrometry), less amount ofnickel by about 1 digit from the portion where a small amount of nickelis added is detected from an amount of the lateral growth portion and itis observed that it diffuses within the amorphous silicon. Further lessamount of nickel by about 1 digit is observed in the amorphous portion.Although the relationship between this fact and the crystalconfiguration is not clear yet, it is possible to form a silicon thinfilm having a crystallinity of desired crystal configuration at adesired section by controlling a nickel adding amount and an addingposition.

Next, electrical characteristics of the nickel micro-added portion wherea small amount of nickel is added and the nearby lateral growth portionwill be explained. Among the electrical characteristics of the nickelmicro-added portion, an electrical conductivity is almost the same withthe film into which no nickel is added, i.e. the film crystallizes atabout 600° C. for tens of hours. When an activation energy is found fromthe temperature dependency of the electrical conductivity, no behaviorconsidered to have been caused by the level of nickel is observed whenthe nickel added amount is 10¹⁷ atoms/cm³ to 10¹⁸ atoms/cm³. As far asthis fact is concerned, it can be concluded that there is no problem inthe operation of TFT if the nickel concentration within the film used inan active layer of TFT and others is less than around 10¹⁸ atoms/cm³.

Contrary to that, the electrical conductivity of the lateral growthportion is higher than that of the nickel micro-added portion by morethan 1 digit, which is considerably high for a silicon semiconductorhaving a crystallinity. This fact is considered to have been caused bythat less or almost no crystal boundaries existed between electrodeswhere electrons (carriers) pass through because the current passingdirection and the crystal lateral growth direction coincide; itcoincides with the result of the pictures of the transmission electronbeam microscope without contradiction. That is, it coincides with theobservation fact that the needle or columnar crystals grow in thedirection parallel to the substrate.

Here, based on the various characteristics described above, an applyingmethod for a TFT will be explained. As an application field of the TFT,an active type liquid crystal display in which TFTs are used for drivingpicture elements will be assumed here.

While it is important to suppress a contraction of the glass substratein the late large screen active type liquid crystal display as describedabove, the use of the nickel micro-adding process of the presentinvention allows to crystallize at a fully lower temperature as compareto the strain point of glass and is especially suitable. The presentinvention allows to replace a conventionally used amorphous silicon withsilicon having a crystallinity by adding a small amount of nickel and bythermally annealing in about 450° to 550° C. for about 4 hours. Althoughit may be necessary to change design rules and others corresponding tothat, it can be fully accommodated with the conventional equipments andprocess and its merit is considered to be great.

Furthermore, the present invention allows to form TFTs used for pictureelements and those forming the drivers of the peripheral circuitseparately utilizing the crystal configurations corresponding to eachcharacteristic and hence is useful when it is applied especially for theactive matrix type liquid crystal display. That is, the TFTs used forthe picture element in the active matrix type liquid crystal display arenot required to have so much mobility and rather than that, there ismore merit for the off current to be smaller. Then in the presentinvention, by directly performing the nickel micro-adding to the regionwhich is to become the TFTs used for the picture element, it is possibleto reduce an off current by growing crystals in a direction vertical toa surface of the substrate and by forming a number of crystal boundariesin a channel direction (a direction when a source region and a drainregion are connected each other by a line). On the other hand,considering to apply the liquid crystal display for a workstation forthe future, a very high mobility is required for the TFTs structuringthe peripheral circuit. Then it is effective to fabricate TFTs having avery high mobility by adding a small amount of nickel near the TFTswhich form the drivers of the peripheral circuit to grow crystals in onedirection (growth in lateral direction) from there and to cause thecrystal growth direction to coincide with the current passing directioninto which carriers move, that is, direction when a source region and adrain region are connected each other by a line).

That is, an object of the present invention is to provide a crystallinesilicon semiconductor film constituting desired TFTs which a crystalgrowth direction is controlled, to selectively fabricate TFTs satisfyingnecessary characteristics on a substrate in a semiconductor device inwhich a large number of thin film transistors are formed on thesubstrate such as a glass substrate.

The feature of the present invention is, in an active matrix type liquidcrystal display having a peripheral circuit portion and a pixel elementportion, to provide TFTs having a crystalline silicon film crystal-grownin a direction vertical to a substrate in the pixel element portion andTFTs having a crystalline silicon film crystal-grown in a directionparallel to a substrate in the peripheral circuit portion. In the pixelelement portion, by using a crystalline silicon film crystal-grown in adirection vertical to a substrate, a structure that carriers movingbetween a source and a drain cross crystal boundaries can be obtained sothat the off current is low in TFTs. On the other hand, in theperipheral circuit portion, TFTs having a high mobility (that is, alarge on current) can be obtained by forming the source and drain inparallel to the crystal growth direction. In operation of TFTs, sincethe carriers flow between the source and drain, possibility which thecarriers cross the crystal boundaries becomes low by forming the sourceand drain in crystal growth direction, therefore, resistance to thecarriers can be reduced.

As described above, the crystal growth direction may be freely selectedin the direction either vertical to the substrate or parallel to thesubstrate by adding a small amount of nickel. Further, the relationshipof the direction into which carriers flow during operation of the TFTand the crystal growth direction may be determined by selecting adirection (direction connecting a source and drain) and position of theTFT to be formed. The direction into which carriers flow described aboveis the direction connecting the source and drain when an insulated gatetype field effect semiconductor device is used for example as the TFT.

The present invention may be used for an active matrix type liquidcrystal display. Further, the TFT having a high mobility may be obtainedby using the crystalline silicon film whose crystal has grown in thedirection parallel to the surface of substrate.

Further, the present invention relates to a fabrication process forobtaining such TFTs as described above. The present invention utilizes atechnology for selectively providing crystallized regions by adding asmall amount nickel.

Although it is typically useful to use nickel as a small amount of metalelement for promoting the crystallization, the similar effect can beobtained even by cobalt, iron and platinum in the present invention.Further, although a kind of substrate is not specifically limited, theusefulness of the present invention that the crystalline silicon filmcan be obtained in a low temperature less than 600° C. as compare to theconventional method become remarkable when it is used for a glasssubstrate and particularly for a large area glass substrate.

While the crystalline silicon film may be thus obtained by selectivelycrystallizing it in a direction vertical or parallel to a surface of asubstrate, the characteristics of such crystalline silicon film may beimproved further by irradiating laser or an equivalent strong lightafter the crystallization process. That is, insufficiently crystallizedcomponents left at the crystal boundaries and others may be crystallizeddue to that. By the way, it is necessary for the region in which the TFTusing the amorphous silicon film is formed not to be irradiated by suchstrong light, because the amorphous silicon is crystallized by theirradiation of such strong light. In this process, characteristics ofboth silicon films crystal-grown in vertical or parallel to the surfaceof the substrate can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing a schematic construction of a liquid crystaldisplay according to an embodiment of the present invention;

FIGS. 2A through 2D are drawings showing a process for fabricating acircuit in which NTFT and PTFT which compose a peripheral circuitsection of the liquid crystal display are formed complementarilyaccording to the embodiment of the present invention;

FIG. 3 is a drawing showing the configuration shown in FIG. 2D seen fromthe above;

FIGS. 4A through 4D are drawings showing a process for fabricating aNTFT formed in a picture element section in the liquid crystal displayaccording to the embodiment of the present invention;

FIGS. 5A through 5E are drawings showing a process for fabricating TFTcircuits in the peripheral circuit section and picture element sectionin the liquid crystal display according to another embodiment of thepresent invention; and

FIGS. 6A and 6B are SEM pictures around the distal end of crystallizedregion of a silicon film crystallized by a growth in a lateral directionin the fabricated TFT.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawings, preferred embodiments of the presentinvention will be explained.

[First Embodiment]

FIG. 1 is a top plan view showing a construction of a liquid crystaldisplay of the embodiment of the present invention in outline, wherein apicture element section 10 having a plurality of picture elementelectrodes provided in matrix (not shown) and a peripheral circuitsection 20 as a driving circuit for driving each of the picture elementelectrodes are shown. According to the present embodiment, thin filmtransistors (TFTs) for driving the picture elements and those composingthe peripheral circuit are formed on an insulated substrate (e.g. aglass substrate). In concrete, the peripheral circuit section is acircuit structured as a CMOS in which P channel type TFT (PTFT) and Nchannel type TFT (NTFT) using silicon films having a crystallinity grownin the lateral direction (called as a crystalline silicon film) areprovided complementarily and the picture element section is TFTs formedas NTFT using silicon films having a crystallinity grown in thelongitudinal direction.

FIGS. 2A to 2D are drawings showing a process for fabricating thecircuit in which the NTFT and PTFT structuring the peripheral circuitsection 20 are formed complementarily. FIGS. 4A to 4D described laterare drawings showing a process for fabricating the NTFT formed on thepicture element section. Because the both fabricating processes areperformed on the same substrate, common processes are executedsimultaneously. That is, the processes shown in FIGS. 2A to 2D and thoseshown in FIGS. 4A to 4D correspond each other, so that they are carriedout in the same time, respectively.

At first, a silicon oxide base film 102 having a thickness of 2000angstrom is formed on a glass substrate (Corning 7059) 101 by asputtering method. A mask 103 formed by a metal mask or silicon oxidefilm is provided only on the peripheral circuit section 20 as shown inFIG. 2A. By the way, because nickel introduced in a later process easilydiffuses also within the silicon oxide film, a thickness of more than1000 angstrom is necessary when the silicon oxide film is used as themask 103. The base film 102 is exposed in a slit shape by the mask 103.That is, seeing the state of FIG. 2A from above, the base film 102 isexposed in the slit shape by a slit shape region 100 while the otherregion is masked. The mask 103 is covered on the whole surface of thepicture element section 10 shown in FIG. 4A and the base film 102 ismasked by the mask 103.

After providing the mask 103, a nickel silicide film (chemical formula:NiSi_(x), 0.4≦×≦2.5, x=2.0 for example) having a thickness of 5 to 200angstrom, e.g. 20 angstrom, is formed by a sputtering method. As aresult, the nickel silicide film is formed over the whole area of theperipheral circuit section 20 and the picture element section 10. Afterthat, the mask 103 is removed to selectively form the nickel silicidefilm only on the region 100. That is, it means that the nickelmicro-adding has been selectively made on the region 100.

Next, after removing the mask 103, an intrinsic (I type) amorphoussilicon film 104 having a thickness of 500 to 1500 angstrom, e.g. 1000angstrom, is deposited by a plasma CVD method. After that, it iscrystallized by annealing for 4 hours at 550° C. under a hydrogenreducing atmosphere (preferably a partial pressure of hydrogen is 0.1 to1 atmospheric pressure). Although the annealing temperature may beselected within a range of about 450° C. to 700° C., a preferablytemperature range is 450° C. to 550° because it takes time for theannealing if the annealing temperature is low and the same result asthat in the prior art is obtained about if the temperature is high. Bythe way, this annealing may be carried out in an inactive atmosphere(e.g. a nitrogen atmosphere) or air.

The silicon film 104 is crystallized in a direction vertical to thesubstrate 101 in the region 100 where the nickel silicide film has beenselectively formed. On the other hand, crystal grows in a lateraldirection (direction parallel to the substrate) from the region 100 asshown by arrow 105 in the peripheral region of the region 100. In thesilicon film 104 in the picture element section 10 (see FIG. 4B) wherenickel silicide film is formed, crystal grows in a direction vertical tothe substrate 101. By the way, a distance of crystal growth in thedirection shown by the arrow 105 which is parallel to the substrate 101is about 40 micron in the above crystal growth.

The amorphous silicon film at the peripheral circuit section 20 may becrystallized by the process described above. Here, the crystal grows inthe lateral direction (direction parallel to the substrate 101) shown inFIG. 2B in the peripheral circuit section 20 and the crystal grows in adirection vertical to the substrate 101 in the picture element section10, as shown in FIG. 4B.

After that, TFTs are separated between the elements, and the siliconfilm 104 at unnecessary part is removed to form an island-shape elementregions. In this process, if a length of an active layer of the TFT(source/drain regions and channel forming region) is within 40 micron,the source/drain regions and channel forming region in the peripheralcircuit portion may be structured by the crystalline silicon film grownin the direction parallel to the substrate 101. Further, if the channelforming region is structured by the crystalline silicon film, the lengthof the active layer may be prolonged further.

Then a silicon oxide film 106 having thickness of 1000 angstrom isformed as a gate insulating film by a sputtering method. Silicon oxideis used as a target in the sputtering. A temperature of the substrateduring the sputtering is 200° to 400° C., e.g. 350° C. Oxygen and argonare used as an atmosphere of the sputtering and a ratio of theargon/oxygen=0 to 0.5, e.g. less than 0.1. Following to that, analuminum film (containing silicon by 0.1 to 2%) having a thickness of6000 to 8000 angstrom, e.g. 6000 angstrom, is formed by a sputteringmethod. By the way, it is desirable to consecutively carry out theprocesses for forming the silicon oxide film 106 and aluminum film.

Gate electrodes 107 and 109 are formed by patterning the formed aluminumfilm. As mentioned above, the process shown in FIG. 2C and that shown inFIG. 4C are carried out simultaneously.

The surface of the gate electrodes 107 and 109 is anodized to form oxidelayers 108 and 110 on the surface thereof. This anodization is carriedout in an ethylene glycol solution containing tartaric acid by 1 to 5%.A thickness of the oxide layers 108 and 110 is 2000 angstrom.

Because the thickness of the oxide layers 108 and 110 is a thickness ofan offset gate region formed in an ion doping process (a process for ionimplanting a doping material) in the later process, a length of theoffset gate region may be determined in the anodizing process.

Next, impurities (phosphorus and boron) are implanted to the siliconregions as regions of elements using the gate electrode 107 andsurrounding oxide layer 108 and the gate electrode 109 and thesurrounding oxide layer 110 respectively as masks. Phosphine (PH₃) anddiborane (B₂ H₆) are used as doping gas. An acceleration voltage is 60to 90 kV, e.g. 80 kV, in the case of phosphine and 40 to 80 kV, e.g. 65kV, in the case of diborane. Dosage is 1×10¹⁵ to 8×10¹⁵ cm⁻², e.g.2×10¹⁵ cm⁻² of phosphorus and 5×10¹⁵ cm⁻² of boron. In the doping, eachelement are selectively doped by covering regions which are notnecessary to be doped by photoresist. As a result, N type impurityregions 114 and 116 and P type impurity regions 111 and 113 are formedand hence, a P channel type TFT (PTFT) region and N channel type TFT(NTFT) region may be formed. Further, a N channel type TFT may be formedin the same time as shown in FIG. 4C.

After that, annealing is carried out by irradiating laser light toactivate the ion implanted impurities. Although KrF eximer laser(wavelength: 248 nm, pulse width: 20 nsec) is used for the laser light,another laser may be used. As laser light irradiating conditions, 2 to10 shots, e.g. 2 shots, of laser light having an energy density of 200to 400 mJ/cm², e.g. 250 mJ/cm², are irradiated per one spot. It isuseful to heat up the substrate to around 200° to 450° C. during theirradiation of laser light. Because nickel has diffused in the regionpreviously crystallized, the recrystallization readily advances byirradiating the laser light in the laser annealing process. Accordingly,the impurity regions 111 and 113 into which the impurity giving P typehas been doped and the impurity regions 114 and 116 into which theimpurity giving N type has been doped may be readily activated.

Following to that, a silicon oxide film 118 having a thickness of 6000angstrom is formed as an interlayer insulator at the peripheral circuitsection 20 as shown in FIG. 2D by a plasma CVD method. After formingcontact holes on the interlayer insulator, electrode and wires 117, 119and 120 of the TFTs are formed by a multi-layered film of titaniumnitride and aluminum. At the picture element section 10, an interlayerinsulator 211 is formed by silicon oxide and after forming contactholes, metal wires 213, 214 and ITO electrode 212 which are used as apicture element electrode are formed as shown in FIG. 4D. Finally, anannealing is carried out for 30 minutes at 350° C. in a hydrogenatmosphere of one atmospheric pressure to complete the TFT circuit orTFTs.

The circuit fabricated as described above has a CMOS structure in whichthe PTFT and NTFT are provided complementarily. However, it is alsopossible to fabricate two independent TFTs simultaneously in the processdescribed above by cutting into two TFTs after fabricating themsimultaneously.

Here, in order to show a positional relationship between the region intowhich nickel has been selectively introduced and the TFTs, a view ofFIG. 2D seen from above is shown in FIG. 3. In FIG. 3, the nickelmicro-adding is selectively performed to the region 100 and crystalgrows in a lateral direction from the location into which nickel hasbeen added by the thermal annealing. The source/drain regions 111 and113 and the channel forming region 112 are formed as the PTFT in thelateral direction to which the crystal grows. Similarly, thesource/drain regions 114 and 116 and the channel forming region 115 areformed as the NTFT.

Because a direction into which carriers flow and the direction ofcrystal growth coincide in the structure described above, the carriersdo not cross the crystal boundary when moving, hence allowing to improvethe operation of the TFTs. For example, a mobility of the PTFTfabricated by the process shown in FIGS. 2A to 2D is 120 to 150 cm² /Vsand it is confirmed that the mobility is improved in comparison with amobility of the prior art PTFT of 50 to 60 cm² /Vs. Further, a mobilityof 150 to 180 cm² /Vs is obtained in the NTFT, which is higher incomparison with a mobility of the prior art NTFT of 80 to 100 cm² /Vs.

Further, a gate insulating film and channel forming region are providedunder the gate electrodes 107 and 109 in FIGS. 2C and 2D. As seen fromFIG. 3, a plurality of TFTs may be simultaneously formed by furtherprolonging (by vertically extending in FIG. 3) the nickel micro-addingregion.

On the other hand, in NTFT formed in the pixel element portion as shownin FIG.4, since a carrier moving (flowing) direction between the source114 and drain 116 is in vertical to a crystal growth direction as shownin 215, carriers cross crystal boundaries when the carriers move.Therefore, the mobility is 30 to 80 cm² /Vs and is characteristic nomore than that of a conventional NTFT. However, when the characteristicof NTFT is examined, it is confirmed that off current is low incomparison with NTFT as shown in FIG. 2. This is an importantcharacteristic when TFT is used as a driver of a pixel elementelectrode. Note that the conventional TFT is a TFT using a crystallinesilicon film crystallized by thermal annealing (600° C. for 24 hours) anamorphous silicon film formed on a glass substrate.

Although the method of selectively forming nickel asthin film (becauseit is extremely thin, it is difficult to observe as a film) on thesurface of the base film 102 under the amorphous silicon film 104 andgrowing the crystal from that portion has been adopted as the method forintroducing nickel, it is also possible to selectively perform thenickel micro-adding after forming the amorphous silicon film 104. Thatis, it is possible to grow the crystal from the top or bottom side ofthe amorphous silicon film. Further, it is also possible to adopt amethod of forming the amorphous silicon film previously and selectivelyimplanting nickel ions into the amorphous silicon film 104 using the iondoping method. This method has a merit that a concentration of thenickel element may be controlled. Nickel micro-adding can be performedby plasma treatment instead of formation of a nickel thin film. Further,it is not always necessary to parallel (or vertically set) the crystalgrowth direction with the direction of the flow of carriers.Characteristics of the TFTs may be controlled by arbitrarily setting anangle between the direction into which carriers flow and the crystalgrowth direction.

[Second Embodiment]

A second embodiment is shown in FIGS. 5A to 5E and FIGS. 6A and 6B.After forming a silicon oxide film 502 having a thickness of 1000 to5000 angstrom, e.g. 2000 angstrom, on a glass substrate 501, anamorphous silicon film having a thickness of 300 to 1500 angstrom, e.g.500 angstrom, is formed by a plasma CVD method. Further, upon that, asilicon oxide film 504 having a thickness of 500 to 1500 angstrom, e.g.500 angstrom, is formed. It is desirable to form those filmsconsecutively. After that, the silicon oxide film 504 is selectivelyetched to form a window region 506 for introducing nickel. The windowregion 506 is formed in a region for fabricating TFTs for a peripheraldriving circuit and the silicon oxide film 504 is removed in the pictureelement section.

Next, a nickel salt film 505 is formed by a spin coating method. Herethe spin coating method will be explained. At first, for the film 505,nickel acetate or nickel nitrate is diluted by water or ethanol. Itsconcentration is 25 to 200 ppm, e.g. 100 ppm.

On the other hand, the substrate is dipped or immersed into a hydrogenperoxide solution or a mixed solution of hydrogen peroxide and ammoniato form a very thin silicon oxide film at the window region 506 and thepixel element section which are a section where the amorphous siliconfilm is exposed to improve the interface affinity of the nickel solutionprepared as described above and the amorphous silicon film.

The substrate treated as such is placed in a spinner and is slowlyrotated. Then 1 to 10 ml, e.g. 2 ml, of nickel solution is dropped onthe substrate to expand the solution over the whole surface of thesubstrate. This state is maintained for 1 to 10 minutes, e.g. 5 minutes.After that, the speed of rotation is increased to carry out spin drying.This operation may be repeated for a plurality of times. Thereby thethin nickel salt film 505 is formed (FIG. 5A).

Here a heat treatment is carried out in a heating furnace within a rangeof 520° to 580° C. and 4 to 12 hours, e.g. at 550° C. for 8 hours. Theatmosphere is nitrogen. As a result, nickel diffuses into the regionright under the window region 506 and the pixel element section andcrystallization starts from this region. A direction of crystallizationis in vertical to the substrate. After that, the crystallized regionexpands into the surrounding area. An expansion direction ofcrystallization is in parallel to the substrate. As a result, threeregions each having different characteristic are formed. A first regionis a region 507 or the pixel element region 510 right under the windowregion 506 and is a region which crystallization proceeds in vertical tothe substrate. A second region is a region 508 which is around theregions 507 and 510 and is a region which crystallization proceeds inparallel to the substrate. On the other hand, the region distant fromthe window region 506 as a third region is not crystallized and remainsas amorphous silicon 509 (FIG. 5B).

After that, the crystallinity is improved further by irradiating KrFeximer laser light (wavelength: 248 nm) or XeCl eximer laser light(wavelength: 308 nm) by 1 to 20 shots, e.g. 5 shots, in air or oxygenatmosphere. The energy density of the laser light is 200 to 350 mJ/cm²and the temperature of the substrate is 200° to 400° C. (FIG. 5C).

After irradiating the laser, the silicon film 503 is etched to form aTFT region of the peripheral circuit and that of the picture elementsection. The region 508 corresponds to the channel forming region of TFTfor the peripheral circuit. Then a silicon oxide film 511 having athickness of 1000 to 1500 angstrom, e.g. 1200 angstrom, is formed andgate electrodes 512, 513 and 514 are formed by aluminum and anodizedfilm thereof similarly to the case of the first embodiment. The gateelectrode 512 is used for a PTFT in the peripheral circuit, gateelectrode 513 is used for an NTFT in the peripheral circuit and gateelectrode 514 is used for a TFT in the picture element section.

Using those gate electrodes as masks, N type and P type impurities areimplanted to the silicon film by an ion doping method similarly to thefirst embodiment. As a result, a source 515, channel 516 and drain 517of the PTFT in the peripheral circuit, a source 520, channel 519 anddrain 518 of the NTFT in the peripheral circuit, a source 521, channel522 and drain 523 of the NTFT in the picture element section are formed.After that, the laser is irradiated on the whole surface to activate thedoped impurities similarly to the first embodiment (FIG. 5D).

A silicon oxide film 524 having a thickness of 3000 to 8000 angstrom,e.g. 5000 angstrom, is formed as an interlayer insulator. Further, anITO film having a thickness of 500 to 1000 angstrom, e.g. 800 angstrom,is formed by a sputtering method and it is pattern-etched to form apicture element electrode 525. Contact holes are formed at thesource/drain of the TFTs, a two layered film of titanium nitrate(thickness: 1000 angstrom) and aluminum (thickness: 5000 angstrom) isdeposited and it is pattern-etched to form electrodes and wires 526through 530. Thus the peripheral circuit can be formed by crystallinesilicon and the picture element section can be formed by amorphoussilicon (FIG. 5E).

According to the present embodiment, laser is irradiated as shown inFIG. 5C to crystallize amorphous components left within the siliconcrystals grown in a needle shape. Further, the needle crystal iscrystallized so that it becomes fat centering on the needle crystal asthe nucleus. It results in expanding a region where current flows andallows for larger drain current to flow.

The silicon film thus crystallized is thinned and then is observed by atransmission type electron microscope (TEM). FIG. 6A is a picture aroundthe distal end of crystallized region of the silicon film crystallizedby the growth in the lateral direction and the needle crystal can beobserved. As seen from FIG. 6A, many non-crystallized amorphous regionsexist among the crystals.

When it is irradiated by the laser under the condition of the presentembodiment, a picture as shown in FIG. 6B is obtained. Although theamorphous regions which have occupied the most of the area in FIG. 6Aare crystallized by this process, an electrical characteristic is not sogood because the crystallized regions are produced at random. Noticed isthe state of crystal in the region which is considered to have beenamorphous among the needle crystals observed around the middle. A fatcrystal region is formed in this region in a manner growing from theneedle crystal (FIG. 6B).

While the pictures in FIGS. 6A and 6B represent the distal end region ofthe crystal where relatively more amorphous regions exist are observedto readily understand the state of the crystal growth, it is the samealso around the nuclei of the crystal and middle of the crystal growth.Thus the amorphous portion can be reduced, the needle crystal can befattened and the characteristics of the TFT can be improved further byirradiating laser.

As described above, the TFTs in the peripheral circuit section arecomposed of the crystalline silicon film whose crystal is grown in thedirection parallel to the flow of carriers and the TFT in the pictureelement section is composed of the crystalline silicon film whosecrystal is grown in the direction vertical to the flow of carriers inthe active matrix type liquid crystal display. Thereby a high speedoperation can be executed in the peripheral circuit section andswitching elements whose off current value is small which are requiredto hold electric charge may be provided in the picture element section.

What is claimed is:
 1. A semiconductor device, comprising:a substrate;and a plurality of thin film transistors formed on the substrate,wherein a part of the plurality of thin film transistors has acrystalline silicon film having a crystal growth direction approximatelyparallel to a surface of the substrate and the other part of theplurality of thin film transistors has a crystalline silicon film havinga crystal growth direction approximately vertical to the surface of thesubstrate.
 2. A semiconductor device, comprising:a substrate; and aplurality of thin film transistors formed on the substrate, wherein apart of the plurality of thin film transistors is provided as aperipheral circuit section of an active matrix type liquid crystaldisplay and the other part of the plurality of thin film transistors isprovided as a picture element section of the active matrix type liquidcrystal display, and the thin film transistors provided as theperipheral circuit section have a crystalline silicon film having acrystal growth direction in a direction parallel to a surface of thesubstrate and the thin film transistors provided as the picture elementsection have a crystalline silicon film having a crystal growthdirection in a direction vertical to the surface of the substrate.
 3. Anactive matrix type liquid crystal display, comprising:a picture elementsection having a plurality of picture element electrodes; and drivingcircuit means for driving each of the picture element electrodes,wherein the picture element section and the driving circuit means arecomposed of thin film transistors each having a substrate, and the thinfilm transistors composing the picture element section have acrystalline silicon film having a crystal growth direction approximatelyvertical to the surface of the substrate and the thin film transistorscomposing the driving circuit means have a crystalline silicon filmhaving a crystal growth direction approximately parallel to the surfaceof the substrate.
 4. A semiconductor device comprising:a substrate; anda crystalline silicon film which is formed on the substrate and has afirst region having a crystal growth direction approximately parallel tothe substrate and a second region having a crystal growth directionapproximately normal to the substrate.
 5. An active matrix type liquidcrystal display comprising:a substrate; a plurality of picture elementelectrodes each having a crystalline silicon region having a crystalgrowth direction approximately vertical to the substrate; and aplurality of driving circuit means for driving the picture elementelectrodes, the plurality of driving circuit means each having acrystalline silicon region having a crystal growth directionapproximately parallel to the substrate, wherein the picture elementelectrodes and the driving circuit means are formed on the substrate. 6.A semiconductor device comprising:a substrate; at least one first thinfilm transistor which is formed on the substrate and has a firstcrystalline silicon region having a crystal growth directionapproximately normal to the substrate; and at least one second thin filmtransistor which is formed on the substrate and has a second crystallinesilicon region having a crystal growth direction approximately parallelto the substrate.
 7. A semiconductor device comprising:a plurality ofthin film transistors each having a crystalline silicon region at leastone first thin film transistor being formed to differ a crystal growthdirection of the crystalline silicon region from a carrier movingdirection of the first thin film transistor and at least one second thinfilm transistor being formed to approximately coincide a crystal growthdirection of the crystalline silicon region with a carrier movingdirection of the second thin film transistor.